1. Field of the Invention
The present invention relates to image processing apparatuses, programs, and storage media.
2. Description of the Related Art
Digital information (digital content) such as digital images can be easily copied by, for example, computers without degrading the information. Recently, as digital image technology has developed, it is also becoming possible to use digital images and the like obtained by, for example, a digital camera as evidential pictures. Some of such digital contents are copyrighted, and copying of them is prohibited without permission or reusing of copied digital content.
Digital content can be easily tampered by performing thereon a simple modifying process, for example, and rewriting them. For this reason, sometimes, digital content is illegally copied and reused without permission, or partially tampered so that the digital contents cannot be used as evidential pictures. Therefore, as for measures to deal with such problems, there are techniques for preventing illegal use of digital contents by methods referred to as digital watermark and data hiding, for example, which methods add into the digital contents (digital images, for example) information that cannot be seen when the digital contents are normally reproduced. With such techniques, based on digital watermarks embedded in the digital contents, it is possible to determine whether digital contents are illegally used.
Applications of such digital watermarks embedded in digital content are as follows.
A. recording of copyright information
B. tracing of illegal copier information
C. recording history of IP address
D. prevention of illegal copy: invisible/high resistance type
E. application for preventing tampering: invisible/low resistance type
F. authentication
G. secret communication
H. embed remarks and labels of digital contents for displaying owner
I. enabling elimination of watermark: visible/reversible type . . . contents distribution
Among the above-mentioned applications, the following technique is devised as the application for realizing “E. application for preventing tampering” which aims at protecting the rights of an auteur of contents.
For example, there is a technique of embedding the serial number of a digital camera and the date and time of photography in a photographed digital image and creating a digital signature, when taking a picture by the digital camera. By utilizing a digital watermark technique in addition to detection of tampering which has been possible by the technology of digital signatures, it is possible to detect whether a digital image is tampered and identify a digital camera that takes a picture and the date and time of photography. Accordingly, it is possible to more effectively prevent tampering of digital images (see, for example, “Investigative Report of Digital Watermark Technology March 1999,” edited by Japanese Electronic Industry Development).
As for technology for embedding a digital watermark in digital contents (hereinafter referred to as “digital watermark technology”), a method is proposed that embeds digital watermark information in spatial frequency data resulting from orthogonal conversion (see Japanese Patent Gazette No. 3109575, for example).
The technology for embedding digital watermark information in spatial frequency data has characteristics by which good effects are obtained when processing and compressing digital contents in which a digital watermark is embedded, though process load of embedding and extracting digital watermark information with respect to digital contents may be heavy.
In recent years, JPEG 2000, which is a new method capable of restoring a high-quality image even at a high compression rate, is being standardized as one image compression method that realizes a high-definition image. According to JPEG 2000, it is possible to perform a compression/decompression process with limited memory by dividing an image into rectangular regions (tiles). That is, each tile constitutes a basic unit when the compression/decompression process is carried out, and a compression/decompression operation can be performed independently for each tile.
However, according to the technology described in Japanese Patent Gazette No. 3109575, the embedding specification such as intensity of digital watermark data is predetermined in a fixed manner. For this reason, even in a case where regions that allow degradation of image quality due to embedding of digital watermark data and regions that do not are mixed, it is impossible to vary the embedding specification of digital watermark data between the regions that allow degradation of image quality and the regions that do not. Hence, there is a problem in that image degradation of partial regions occurs as a result of embedding of digital watermark data in digital contents.